Toxoplasma gondii is an obligate intracellular pathogen that is a major cause of disease in AIDS patients and other immune compromised individuals. There are no vaccines to prevent infections and anti-Toxoplasma drugs are poorly tolerated and cannot eliminate the parasite. Thus, new drug targets are needed to improve treatment. Like other obligate intracellular pathogens, Toxoplasma must co-opt host cell proteins and pathways. However, few host pathways needed for Toxoplasma growth are known. Identifying these host pathways is critical because these pathways and the parasite factors that regulate them are novel drug targets. Because Toxoplasma is a eukaryote, host cell pathways regulated by the parasite cannot be identified using pharmacological inhibitors because these drugs would likely inhibit not only host proteins but also parasite ones. RNA interference (RNAi) is not subject to this limitation. Thus, we will use a siRNA screen to identify host cell proteins critical for parasite growth. We validated this approach by first demonstrating that parasite growth is inhibited in cells transfected with siRNAs against a host gene known to be necessary for parasite growth. Second, a proof of principle screen led to our discovery of a critical role for host microtubules in parasite invasion. To extend these findings, two specific aims are proposed. In Specific Aim 1, we will complete screening our libraries and identify siRNAs that significantly and specifically inhibit Toxoplasma growth. In Specific Aim 2, we will determine i) when during the parasite's lytic growth cycle does a host protein function, ii) how infection influences the expression and localization of these host proteins, and ii) if a host protein is differentially regulated by virulent and avirulent Toxoplasma strains. Together, these data will identify novel host pathways critical for Toxoplasma growth, which is an important first step in developing new treatments.